U.S. Pat. No. of Jablonowski, 4,286,293 issued Aug. 25, 1981 (assigned to a subsidiary of the present assignee company) is directed to a method for inspecting gold conductors on ceramic substrates by scanning the substrate surface with a laser beam and detecting light reflected and scattered from the surface for display on a video monitor. It was noted that light from the gold conductors was reflected at a relatively small angle with respect to a normal to the surface while light scattered from the ceramic surface was predominantly scattered at a high angle with respect to the surface normal. The patent teaches that greater contrast on the video display between the gold conductors and the ceramic substrates can therefore be obtained by subtracting the light scattered at a high angle from the light received at a relatively low angle to the normal. Thus, the patent teaches the use of two optical fiber waveguides for directing light to two detectors and combining subtractively the detector outputs.
The U.S. Pat. No. of Heebner et al. 4,441,124, issued Apr. 3, 1984 (issued to a subsidiary of the present assignee company) teaches how the principles of the Jablonowski patent can be used for silicon wafer inspection, particularly for detecting the presence of spurious particulates. For this purpose, the light which is displayed on a monitor is that which is scattered at a high angle with respect to the wafer surface normal. To efficiently gather this light, a ring defined by the ends of light guides or optical fibers is arranged around the periphery of the wafer to be inspected. The presence of a particulate is then manifested by an increase of scattered light detected by these optical fibers arranged essentially coplanar with the surface of the wafer (i.e., at a high angle of nearly 90 degrees with respect to the wafer surface normal). In order to provide greater contrast for the display, light scattered and reflected at a relatively low angle with respect to the surface normal is also detected and may be subtracted from the signal generated by the light scattered at a high angle with respect to the normal. Thus, to emphasize light scattered at a high angle with respect to the surface normal, Heebner teaches subtracting light scattered at a low angle; whereas, in Jablonowski, to emphasize light scattered at a low angle, light scattered at a high angle is subtracted.
In adapting the Heebner technique to the inspection of alumina substrates, we have determined that enhancing contrast through a subtraction of light reflected at a low angle is not necessary. However, we have also determined that there is a need to define more accurately the nature of any defects on the surface of alumina substrates being inspected. Specifically, it would be useful to be able to define with some accuracy the dimensions of any defect detected and its topography.